More than 1.5 million years ago, several human ancestors wandered across a mud flat at what is now Ileret, Kenya. The footprints they left behind, the second oldest ever found, reveal that these early humans had evolved big, modern feet--and that they walked just like we do, according to a new study.

Human ancestors began walking upright at least 6 million years ago, according to analysis of hominid leg and pelvic bones[2]. But researchers have debated when they evolved the ability to walk upright in a modern manner, rather than with a more primitive gait, possibly like the bent-kneed waddle of chimpanzees. Footprints found in Laetoli, Tanzania, show that the australopithecines that made them 3.75 million years ago had longer toes, a shallower arch, and a more apelike big toe that jutted slightly away from the other toes. This suggested to some that they had a more primitive gait and that the transition to fully modern walking didn't happen until our direct ancestor, Homo erectus, emerged about 1.9 million years ago. However, researchers had few fossils of the foot of H. erectus to prove it walked just like we do. Now, with the discovery of the footprints, which were probably made by H. erectus, at Ileret, they have direct evidence of how it walked.

To find out, a team led by Matthew Bennett of Bournemouth University in the United Kingdom scanned and digitized at least four trails of footprints laid down over several thousand years at Ileret. The researchers were able to use the size, spacing, and depth of the impressions to estimate the weight, stride length, and gait of the ancient walkers. As the researchers report in tomorrow's issue of Science[3], the new footprints show that these early humans were pushing off the ground with their big toes--or toeing off--and shifting their weight over these digits in the same way as modern humans. H. erectus's feet had clearly evolved a modern shape, with the big toe parallel to the other toes and a pronounced arch, says paleoanthroologist Brian Richmond of George Washington University in Washington, D.C.

In addition to finding the human footprints, the researchers also found tracks of ancient animals, from birds to lions, that provide a "snapshot in time of what animals were on the landscape," says paleoanthropologist and co-author John Harris of Rutgers University, New Brunswick. The new glimpse of the footpaths of animals and humans complement earlier studies that reveal the anatomy and behavior of H. erectus, suggesting that as it evolved modern body proportions, it also increased its home range and began competing with carnivores for carcasses on the savanna, says Harris.

Other scientists agree that the prints expose the steps our ancestors took on the way to becoming modern. "Fossil footprints are literally frozen behavior," says anatomist William Jungers of Stony Brook University Medical Center in New York state. "They confirm that a modern, humanlike, bipedal gait is present by at least 1.5 million years ago, with all the biomechanical nuances we associate with our own way of walking." But the shoulder and pelvis of H. erectus were still primitive, says Jungers, so the footprints also show that on the path to modernity, "the foot led the way."